Plasma has long been employed to process substrates, including for example semiconductor wafers for integrated circuit fabrication and panels for liquid crystal display fabrication. As technology improves and more and more devices are packaged into a smaller area of the substrate, it is critical that the processing precisely forms these devices and minimizes defects.
A critical parameter for plasma processing of substrates nowadays is azimuthal uniformity. Azimuthal uniformity refers to the uniformity of devices or features formed around the wafer. For example, given a certain reference angle on a wafer, it is desirable that the devices formed at a 45-degree position from that reference angle be essentially the same as the devices formed at 135 degrees, 180 degrees, or 210 degrees from that reference angle.
Azimuthal uniformity is especially challenging in some modern plasma processing chambers because of the non-symmetry of some components within the chamber. For example, in some chambers, the lower electrode assembly is supported by a cantilever arm. With this arrangement, the RF return current is asymmetric due to the presence of the cantilever arm in one part of the chamber.
The cantilever arm also affects azimuthal non-uniformity in other ways. For example, exhaust gas has to flow around the cantilever arm as it is pumped from the plasma generating region to the exhaust port, which is typically located at the bottom of the chamber. The non-uniformity of the gas flow due to the presence of the cantilever arm affects pressure uniformity in the azimuthal direction, which in turn affects the plasma density in the azimuthal direction, thereby affecting process uniformity in the azimuthal direction.
As another example, certain components may be placed in certain locations around the chamber but not at others. For example, an OES (optical emission spectroscopy) port may exist in one location of the chamber liner but not in another. The presence of such non-symmetric components introduces azimuthal non-uniformity with respect to the RF current return path(s), which may result in non-uniform processing around the wafer in the azimuthal direction.
As modern processing requirements become more stringent, many customers have specified that azimuthal uniformity be below, for example, one percent or even lower. Accordingly, there is a need to monitor the azimuthal non-uniformity for development and processing purposes.